You underestimate corporate R&D time... Most likely, both the 870M and 590M have been in development for years, and probably started development at roughly the same time, for the same reasons. Not really surprising that both manufacturers released their solution with a few months of each other.
The explosion of 3-gun has really driven the market for a reliable, detachable mag shotgun. The Turk guns (yes, they're available in the US), and the AK pattern shotguns have both been available for a while now, and just aren't gaining traction on the competition circuit - because they just don't meet the reliability requirement of the higher end shooters. Miculek uses an AK pattern shotgun, sometimes, but not often - it's failed pretty badly on him in matches.
The tactical crowd will buy them, sure, but I'm waiting to see what starts dominating the 3-gun circuit. Sponsorship or not, the top level competitors won't use it unless they KNOW it's 100% reliable.